One of the fundamental principles of physics is the relationship between electricity and magnetism. This relationship was first observed in the mid-1800s when it was noted that current passing through a simple bar conductor, induces a magnetic field perpendicular to the direction of current flow. As a result of the induced magnetic field, each of the moving charges, which comprises the current, experiences a force. The force exerted on each of the moving charges generates torque on the conductor proportional to the magnetic field.
It is a well understood aspect of electrometric theory that as current passes through a simple bar conductor, it induces a magnetic field perpendicular to the direction of current flow. As a result of the induced magnetic field, each of the moving charges comprising the current, experiences a force. The force exerted on each of the moving charges generates torque. It is this principle that underpins devices such as electric motors and generators.
Most typical DC motors consist of three main components namely a stator, armature/rotor and commutator. The stator typically provides a magnetic field which interacts with the field induced in the armature to create motion. The commutator acts to reverse the current flowing in the armature every half revolution thereby reversing the field in the armature to maintain its rotation within the field in the one direction. A DC motor in its simplest form can be described by the following three relationships:ea=KΦωV=ea+Raia T=KΦia Where ea is the back emf, V the voltage applied to the motor, T the torque, K the motor constant, Φ the magnetic flux, ω the rotational speed of the motor, Ra the armature resistance and ia the armature current.
The magnetic field in a typical motor is stationary (on the stator) and is created by permanent magnets or by coils. As current is applied to the armature/rotor, the force on each conductor in the armature is given by F=ia×B×1. Back emf is generated due to a relative rate of flux change as a result of the conductors within the armature rotating through the stationary field. The armature voltage loop therefore contains the back emf plus the resistive losses in the windings. Thus, speed control of the DC motor is primarily through the voltage V applied to the armature while torque scales with the product of magnetic flux and current.
Thus, in order to maximise torque in a DC motor, one would presume that it is simply a matter of increasing either the magnetic field or the current supplied. In practice, however, there are limitations. For instance, the size of the magnetic field which can be generated via permanent magnets is limited by a number of factors. In order to produce a significantly large field from a permanent magnet, the physical size of the magnet is relatively large (e.g. a 230 mm N35 magnet is capable of producing a field of a few Kilogauss (kG)). Significantly, larger fields can be produced utilising a plurality of magnets, the size and number of magnets again adds to the overall size and weight of the system. Both size and weight of the motor are critical design considerations in applications such as electric propulsion systems. Generation of larger magnetic fields is possible utilising standard wire coils but the size, weight and heating effects make the use of standard coils impractical.
Another factor which has an effect on torque that needs consideration is the production of drag caused by eddy currents created within the armature/rotor. Eddy currents occur where there is a temporal variation in the magnetic field, a change in the magnetic field through a conductor or change due to the relative motion of a source of magnetic field and a conducting material. The eddy currents induce magnetic fields that oppose the change of the original magnetic field per Lenz's law, causing repulsive or drag forces between the conductor and the magnet. The power loss (P) caused by eddy currents for the case of a simple conductor assuming a uniform a material and field, and neglecting skin effect can be calculated by:
  P  =                    π        2            ⁢              B        P        2            ⁢              d        2            ⁢              f        2                    12      ⁢                          ⁢      ρ      ⁢                          ⁢      D      where Bp is peak flux density, d—thickness or diameter of the wire, ρ—resistivity, σ—electrical conductivity, μ magnetic permeability, f frequency (change in field) and penetration depth (D).
As can be seen from the above equation, as the magnetic field increases the size and effects of eddy currents increase i.e. the higher the magnetic field, the greater the drag produced as a result of eddy currents. In addition to the field strength, the resistivity of and thickness of the conductive elements in the armature are also a factor. Selection of the material of the conductive elements in the armature can greatly affect the amount of current that can be applied to the armature.
These basic properties and functions are the focus of continuing developments in the search for improved devices having better efficiencies.
It will be clearly understood that if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.